This invention relates generally to electronic switches, and particularly to those which lend themselves to use as bidirectional, or alternating current (AC), switches for on-off control of a variety of electric circuits notably including matrix converters and AC power supplies.
Perhaps the most familiar example of AC switches is a bidirectional triode thyristor, better known as TRIAC (TRIode for Alternating Current). The TRIAC has the weakness that, once triggered, it remains conductive until the current flowing therethrough drops below a predetermined threshold voltage. It cannot possibly be turned off at a controllable point in an AC cycle.
Some bidirectional switching devices have been known which meet that requirement. One of them (shown in FIG. 1 of the drawings attached hereto) comprises two insulated-gate bipolar transistors (IGBTs) connected in inverse series with each other and two diodes connected in inverse parallel with the respective IGBTs. The two IGBTs are replaceable with insulated-gate field-effect transistors (IGFETs), junction gate field-effect transistors (JFETs), or bipolar transistors. A further known bidirectional switch is an inverse parallel connection of two IGBTs.
These prior art bidirectional switching circuits, all incorporating normally-off solid-state switches, are alike in being unnecessarily complex in construction and high in turn-on resistance and voltage. Moreover, being made from semiconducting silicon materials with relatively low bandgaps, such known devices are not so high in antivoltage strength as can be desired.
WO 2004/114508 (FIG. 2) teaches a bidirectional switching device expressly designed to defeat all the noted drawbacks of the more conventional devices above. This prior art device offers the benefits of greater simplicity in construction and a higher antivoltage strength, the latter being a result of the fact that its semiconductor switch, typically in the form of a high electron mobility transistor (HEMT), is made from a semiconducting compound such as, usually, nitride.
Offsetting these benefits is the fact that this prior art device as well as its HEMT switch is normally on, necessitating use of a complex and expensive gate control circuit capable of applying a negative potential to the gate terminal of the HEMT in order to turn it off. The prior art device also requires an overcurrent protector for precluding the flow of an excessive current when the circuit is powered on. Incidentally, as far as this applicant is aware, nitride and other compound semiconductor devices are still a fledgling art. It is difficult to make the HEMT switch of this prior art bidirectional switching device normally off with the semiconductor materials technology available today.